1. Field of the Invention
This invention relates to a seismic vibrator for use in injecting swept-frequency acoustic signals into the earth. More particularly, the invention is concerned with automatically stabilizing the applied force level by means of an adaptive preset signal that is applied at the beginning of a sweep.
2. Discussion of the Prior Art
A seismic vibrator typically consists essentially of a ground-contacting base plate that is driven in reciprocal strokes by an electro-hydraulic servo-controlled linear actuator which acts against a free-floating inertia mass.
Customarily the vibrator mechanism is mounted on a heavy vehicle weighing on the order of 40,000 to 65,000 pounds. In operation, a portion of the mass of the truck is applied to the base plate, through a compliant means, as a static bias loading. That loading is required in order to couple the vibrator base plate to the ground. When the linear actuator is operated to cause the base plate to shake the ground, a certain force is developed. The applied force level is dependent upon the drive level of the system itself and upon the ground impedance, which varies considerably from place to place. If the force level exceeds the static bias loading, the base plate becomes decoupled from (jumps off) the ground, a most undesirable situation.
Manual adjustments may be made to the drive level. But such adjustments are necessarily empirical and may not be opitmal for all environments. Therefore, automatic feedback circuitry has been developed to minimize or eliminate unwanted base-plate decoupling.
One such system is taught by U.S. Pat. No. 4,184,144, issued Jan. 15, 1980 to Rickenbacker, which is incorporated herein by reference. Another patent of interest is U.S. Pat. No. 3,208,550 issued Sept. 28, 1965, to Castanet et al. In U.S. Pat. No. 4,184,144, the instantaneous force levels developed at the inertia mass and the base plate are measured, combined, and processed to generate a controlled signal that is the envelope of the peak level of the base plate ground force less the static force. That signal is fed back for the purpose of modulating the swept-frequency input drive signal as required, to prevent base-plate decoupling. More precisely the purpose is to maintain the controlled signal at some desired force level that is less than the static force.
The usual type of vibrator drive signal is a swept-frequency chirp signal having a frequency range of, perhaps, 10-125 Hz and having a finite time duration of four to eight or more seconds. For all practical purposes, the input sweep signal (or simply "sweep" for short) has a constant peak amplitude except for the beginning and ending portions of the sweep signal where the peak amplitudes are tapered up and tapered down respectively. The controlled signal is fed back to circuits that generate a control signal that then operates on the input drive signal so as to automatically maintain a force level beneath the base plate that is equal to or less than the static loading.
For the purposes of this disclosure, the term "vibrator" includes the vehicle, base plate, linear actuator etc., such that the individual parts need not necessarily be recited. The term "sweep generation" means the act of driving the vibrator base plate by the linear actuator in response to an input sweep signal, for the purpose of shaking the ground.
At the beginning of a sweep signal, the system has no history as to the correct initial drive level other than that provided by a gross manual drive-level setting. Hence during the initial portion of a sweep, the system tends to oscillate uncontrollably before stabilizing. Since with any servo loop, there is an inherent time lag between sensing of an overdrive condition and correction thereof, the initial portion of the sweep may occupy as much as a half-second or more. However during the first fraction of a second, the servo loop should act so as to drive the loop in the direction of stabilization.
Earlier, I pointed out that the ground impedance varies considerably from place to place. However between a series of stations that are not very far apart, such as 100-300 feet, the impedance change is usually gradual. Therefore it is reasonable to assume that the control-signal levels during sweeps at adjacent stations would be similar. Thus, a control-signal sample taken during or just after the end of the initial period of a prior sweep should approach the correct value so that it could be used as a preset control signal during the initial period of a subsequent sweep.
It is a purpose of this invention to superimpose an adaptive preset signal on a control-signal feedback circuit during the initial portion of a sweep.